1. Field of the Invention
The present invention relates to silicone grease compositions and to a method for preparing these compositions. More specifically, this invention relates to silicone grease compositions exhibiting little oil bleed even upon long-term standing at elevated temperatures. The present invention also relates to a method for preparing these compositions utilizing shear-induced liquefaction of a crosslinked organosiloxane gel.
2. Background Information
Silicone grease compositions prepared by blending a thickener such as finely divided silica, diatomaceous earth, zinc oxide, or titanium oxide into liquid organosiloxanes referred to as silicone oils are distinguished by an excellent water repellency, resistance to moisture, dust and corrosion, lubrication performance, sealing capacity, and electric insulation performance. As a consequence, they have been used as lubricants and corrosion inhibitors for rubbing contacts such as switches and tuners; as insulating agents, water repellents, and lubricants for cable connectors; and to inhibit damage to electric insulators resulting from salt and dust.
In addition, silicone grease compositions prepared by blending a thickener such as alumina, boron nitride, or aluminum nitride into silicone oil have an excellent thermal conductivity and as a consequence are employed as heat-conducting fillers between a heat-generating solid state electronic device such as transistor, diode, or rectifier and a heat-radiating plate.
The preceding silicone grease compositions are prepared by blending a silicone oil with a thickener which has little affinity for the silicone oil. As a result, the silicone oil will separate or bleed from such a silicone grease composition when it is held for extended periods of time at high temperatures or when it is repeatedly subjected to alternating cooling and heating cycles over long periods of time.
Oil separation is a particularly frequent problem associated with the use of the highly thermally conductive thickeners that exhibit relatively large particle sizes. This problem has prompted a number of investigations into low-bleed silicone grease compositions, and the following compositions, for example, have been described:
a silicone grease composition composed of straight-chain and/or cyclic organopolysiloxane and thickener in Japanese Patent Application Laid Open [Kokai or Unexamined] Number 51-55870 [55,870/76]), PA1 a thixotropic, thermally conductive material composed of liquid organosilicone carrier, filler powder which imparts thermal conductivity (selected from lamellar aluminum nitride, dendritic zinc oxide, lamellar boron nitride, and their mixtures), and silica fiber functioning as bleed inhibitor, in Japanese Laid Open Patent Application 57-36302 [36,302/82], PA1 a thermally conductive silicone grease composition composed of polyorganosiloxane, silicon carbide, and fumed silica, in Japanese Laid Open Patent Application Number 62-43492 [43,492/87], and PA1 a thermally conductive silicone oil compound composed of hydroxyl-containing organopolysiloxane and at least 1 microparticulate metal compound selected from zinc white, alumina, aluminum nitride, and silicon nitride, in Japanese Patent Application Laid Open Number 2-212556 [212,556/90].
In another approach to the preparation of silicone grease compositions, U.S. Pat. No. 4,987,169, issued on Jan. 22, 1991 teaches preparing a silicone composition in powder, paste, or grease form by the application of a shearing force to the polymer product obtained by the addition polymerization in the presence of a low-viscosity silicone oil of an organohydrogenpolysiloxane with organopolysiloxane containing silicon-bonded, ethylenically unsaturated groups.
The silicone grease compositions disclosed in both Japanese Patent Publication Number 57-36302 and Japanese Laid Open Patent Application Number 62-43492 exhibit relatively low thermal conductivities. This is due to the addition of silica fiber or fumed silica, which requires a reduction in the amount of addition of the highly thermally conductive filler that can be present in the composition.
Even though the silicone grease compositions disclosed in Japanese Patent Application Laid Open Numbers 51-55870 and 2-212556 employ special organopolysiloxanes, these compositions still do not offer a satisfactory reduction in the oil bleed or oil separation.
Finally, the preparative method described in U.S. Pat. No. 4,987,169 has the disadvantage of a large oil bleed due to the presence of the low-viscosity silicone oil in the final composition.
The present invention was developed as the result of extensive investigations by the present inventors for the purpose of solving the problems associated with prior art grease compositions.
An objective of the present invention is to provide a silicone grease composition wherein the amount of oil that bleeds or separates from the composition is equivalent to less than 1 percent by weight of the composition even upon standing at high temperatures for long time intervals. A second objective is to provide a method for the preparation of this silicone grease composition.